Outline • Pinhole camera •Film camera • Digital camera Cameras • Video camera Digital Visual Effects, Spring 2007 Yung-Yu Chuang 2007/3/6 with slides by Fredo Durand, Brian Curless, Steve Seitz and Alexei Efros Camera trial #1 Pinhole camera pinhole camera scene film scene barrier film Add a barrier to block off most of the rays. • It reduces blurring Put a piece of film in front of an object. • The pinhole is known as the aperture • The image is inverted Shrinking the aperture Shrinking the aperture Why not making the aperture as small as possible? • Less light gets through • Diffraction effect High-end commercial pinhole cameras Adding a lens “circle of confusion” scene lens film A lens focuses light onto the film $200~$700 • There is a specific distance at which objects are “in focus” • other points project to a “circle of confusion” in the image Lenses Exposure = aperture + shutter speed F Thin lens equation: • Aperture of diameter D restricts the range of rays (aperture may be on either side of the lens) • Any object point satisfying this equation is in focus • Shutter speed is the amount of time that light is • Thin lens applet: allowed to pass through the aperture http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html Exposure Effects of shutter speeds • Two main parameters: • Slower shutter speed => more light, but more motion blur – Aperture (in f stop) – Shutter speed (in fraction of a second) • Faster shutter speed freezes motion Aperture Depth of field • Aperture is the diameter of the lens opening, usually specified by f-stop, f/D, a fraction of the focal length. – f/2.0 on a 50mm means that the aperture is 25mm – f/2.0 on a 100mm means that the aperture is 50mm •When a change in f-stop occurs, the light is either doubled or cut in half. • Lower f-stop, more light Changing the aperture size affects depth of field. (larger lens opening) A smaller aperture increases the range in which • Higher f-stop, less light the object is approximately in focus (smaller lens opening) See http://www.photonhead.com/simcam/ Exposure & metering Pros and cons of various modes • The camera metering system measures how • Aperture priority bright the scene is – Direct depth of field control • In Aperture priority mode, the photographer sets – Cons: can require impossible shutter speed (e.g. with f/1.4 for a bright scene) the aperture, the camera sets the shutter speed • Shutter speed priority • In Shutter-speed priority mode, photographers – Direct motion blur control sets the shutter speed and the camera deduces – Cons: can require impossible aperture (e.g. when the aperture requesting a 1/1000 speed for a dark scene) • In Program mode, the camera decides both • Note that aperture is somewhat more restricted exposure and shutter speed (middle value more • Program or less) – Almost no control, but no need for neurons • In Manual mode, the user decides everything • Manual (but can get feedback) – Full control, but takes more time and thinking Distortion Correcting radial distortion No distortion Pin cushion Barrel • Radial distortion of the image – Caused by imperfect lenses – Deviations are most noticeable for rays that pass through the edge of the lens from Helmut Dersch Film camera Digital camera aperture aperture & shutter & shutter scene lens & film scene lens & sensor motor motor array • A digital camera replaces film with a sensor array • Each cell in the array is a light-sensitive diode that converts photons to electrons CCD v.s. CMOS Sensor noise • CCD is less susceptible to noise (special process, higher • Blooming fill factor) • CMOS is more flexible, less expensive (standard • Diffusion process), less power consumption •Dark current • Photon shot noise • Amplifier readout noise CCD CMOS SLR (Single-Lens Reflex) SLR view finder • Reflex (R in SLR) means that we see through the same lens used to take the image. Prism • Not the case for compact cameras Your eye Mirror (flipped for exposure) Film/sensor Mirror (when viewing) Light from scene lens Color Field sequential So far, we’ve only talked about monochrome sensors. Color imaging has been implemented in a number of ways: • Field sequential • Multi-chip • Color filter array •X3 sensor Field sequential Field sequential Prokudin-Gorskii (early 1900’s) Prokudin-Gorskii (early 1990’s) Lantern projector http://www.loc.gov/exhibits/empire/ Multi-chip Embedded color filters wavelength dependent Color filters can be manufactured directly onto the photodetectors. Color filter array Why CMY CFA might be better Kodak DCS620x CMY Color filter arrays (CFAs)/color filter mosaics Color filter array Bayer’s pattern Bayer pattern Color filter arrays (CFAs)/color filter mosaics Demosaicking CFA’s Demosaicking CFA’s bilinear interpolation Constant hue-based interpolation (Cok) Hue: Interpolate G first original input linear interpolation Demosaicking CFA’s Demosaicking CFA’s Median-based interpolation (Freeman) Median-based interpolation (Freeman) 1. Linear interpolation 2. Median filter on color original input linear interpolation differences color difference median filter Reconstruction (e.g. G-R) (kernel size 5) (G=R+filtered difference) Demosaicking CFA’s Demosaicking CFA’s Gradient-based interpolation Gradient-based interpolation (LaRoche-Prescott) (LaRoche-Prescott) 1. Interpolation on G 2. Interpolation of color differences Demosaicking CFA’s Demosaicking CFA’s bilinear Cok Freeman LaRoche Generally, Freeman’s is the best, especially for natural images. Foveon X3 sensor Color filter array • light penetrates to different depths for different wavelengths • multilayer CMOS sensor gets 3 different spectral sensitivities red green blue output X3 technology Foveon X3 sensor Bayer CFA X3 sensor red green blue output Cameras with X3 Sigma SD9 vs Canon D30 Sigma SD10, SD9 Polaroid X530 Color processing White Balance • After color values are recorded, more color processing usually happens: – White balance – Non-linearity to approximate film response or match TV monitor gamma warmer +3 automatic white balance Manual white balance Autofocus •Active –Sonar –Infrared •Passive white balance with white balance with the white book the red book Digital camera review website Camcorder • Demonstration of digital cameras • http://www.dpreview.com/ • A cool video of digital camera illustration Interlacing Deinterlacing without interlacing with interlacing blend weave Deinterlacing References • http://www.howstuffworks.com/digital-camera.htm • http://electronics.howstuffworks.com/autofocus.htm • Ramanath, Snyder, Bilbro, and Sander. Demosaicking Methods for Bayer Color Arrays, Journal of Electronic Imaging, 11(3), pp306-315. • Rajeev Ramanath, Wesley E. Snyder, Youngjun Yoo, Mark S. Drew, Color Image Processing Pipeline in Digital Still Cameras, IEEE Signal Processing Magazine Special Issue on Color Image Processing, vol. 22, no. 1, pp. 34- 43, 2005. Discard • http://www.worldatwar.org/photos/whitebalance/ind Progressive scan ex.mhtml (even field only or odd filed only) • http://www.100fps.com/.